NOx emissions from diesel engines are an environmental problem. Several countries, including the United States, have long had regulations pending that will limit NOx emissions from trucks and other diesel-powered vehicles. Manufacturers and researchers have put considerable effort toward meeting those regulations.
In gasoline powered vehicles that use stoichiometric fuel-air mixtures, three-way catalysts have been shown to control NOx emissions. In diesel-powered vehicles, which use compression ignition, the exhaust is generally too oxygen-rich for three-way catalysts to be effective.
Several solutions have been proposed for controlling NOx emissions from diesel-powered vehicles. One set of approaches focuses on the engine. Techniques such as exhaust gas recirculation and partially homogenizing fuel-air mixtures are helpful, but these techniques alone will not eliminate NOx emissions. Another set of approaches remove NOx from the vehicle exhaust. These include the use of lean-burn NOX catalysts, selective catalytic reduction (SCR) catalysts, and lean NOX traps (LNTs).
Lean-burn NOx catalysts promote the reduction of NOx under oxygen-rich conditions. Reduction of NOx in an oxidizing atmosphere is difficult. It has proven challenging to find a lean-burn NOx catalyst that has the required activity, durability, and operating temperature range. A reductant such as diesel fuel must be steadily supplied to the exhaust for lean NOx reduction, introducing a fuel economy penalty of 3% or more. Currently, peak NOx conversion efficiencies for lean-burn NOx catalysts are unacceptably low.
SCR generally refers to selective catalytic reduction of NOx by ammonia. The reaction takes place even in an oxidizing environment. The NOx can be temporarily stored in an adsorbent or ammonia can be fed continuously into the exhaust. SCR can achieve high levels of NOx reduction, but there is a disadvantage in the lack of infrastructure for distributing ammonia or a suitable precursor. Another concern relates to the possible release of ammonia into the environment.
To clarify the state of a sometimes ambiguous nomenclature, one should note that in the exhaust aftertreatment art the terms “SCR catalyst” and “lean NOx catalyst” can be used interchangeably. Often, however, the term “SCR” is used to refer just to ammonia-SCR, in spite of the fact that strictly speaking ammonia-SCR is only one type of SCR/lean NOx catalysis. Commonly, when both ammonia-SCR catalysts and lean NOx catalysts are discussed in one reference, SCR is used in reference to ammonia-SCR and lean NOx catalysis is used in reference to SCR with reductants other than ammonia, such as SCR with hydrocarbons.
LNTs are devices that adsorb NOx under lean exhaust conditions and reduce and release the adsorbed NOx under rich conditions. An LNT generally includes a NOx adsorbent and a catalyst. The adsorbent is typically an alkaline earth compound, such as BaCO3 and the catalyst is typically a combination of precious metals including Pt and Rh. In lean exhaust, the catalyst speeds oxidizing reactions that lead to NOx adsorption. In a reducing environment, the catalyst activates reactions by which hydrocarbon reductants are converted to more active species, the water-gas shift reaction, which produces more active hydrogen from less active CO, and reactions by which adsorbed NOx is reduced and desorbed. In a typical operating protocol, a reducing environment will be created within the exhaust from time-to-time to regenerate (denitrate) the LNT.
An LNT can produce ammonia during denitration. Accordingly, it has been proposed to combine LNT and ammonia-SCR catalysts into one system. Ammonia produced by the LNT during regeneration is captured by the SCR catalyst for subsequent use in reducing NOx, thereby improving conversion efficiency over a stand-alone LNT with no increase in fuel penalty or precious metal usage. U.S. Pat. No. 6,732,507 describes such a system. U.S. Pat. Pub. No. 2004/0076565 describes such systems wherein both components are contained within a single shell or disbursed over one substrate.
An SCR catalyst can be used to address the problem of ammonia release from the LNT during regeneration, but there is another issue in that some NOx is released without being reduced. The release occurs primarily at the beginning of LNT regeneration. The resulting sharp and transient increase in exhaust NOx concentration is often referred to as an NOx release spike. Several theories have been proffered to explain this release spike. These theories have led to diverse proposals for potential solutions.
U.S. Pat. Pub. No. 2004/0076565 proposes that the NOx spike results from a sudden increase in LNT temperature due to reaction of reductant with oxygen stored in the LNT. The proposed solution is to reduce the oxygen storage capacity of the LNT.
WO 2005/049984 proposes that the NOx spike results from violent reactions between oxygen-containing exhaust gases and reductant rich exhaust gases mixing within the interstices of the LNT at the beginning of the regeneration. The proposed solution is a near stoichiometric phase in between rich and lean phases. Oxygen carrying exhaust gas is to be flushed from the LNT during the near stoichiometric phase by an exhaust gas that contains little or no reductant.
U.S. Pat. No. 5,740,669 proposes that the NOx spike results from the exhaust conditions occurring within the LNT during the transition period between lean and rich phases. During the transition period, the exhaust is though to be sufficiently rich to cause NOx to release, but not sufficiency rich to reduce all the released NOx. The proposed solution is to regenerate the LNT only when the LNT is below a predetermined temperature, whereby NOx is not so readily released.
U.S. Pat. No. 5,778,667 suggests that the NOx spike results from an imbalance between the rate of release of NOx and the availability of HC and CO reductants. The proposed solution is to introduce ammonia, which is used to reduce the released NOx downstream from the NOx absorber.
U.S. Pat. No. 6,718,756 suggests that the NOx spike is caused by CO in the exhaust, which both releases NOx and reduces NOx, but at rates that do not match. It is said that increasing the CO supply rate will not ameliorate the spike, because CO increases the release rate as well as the reduction rate. The proposed solution is to supply a reductant that does not cause NOx release. The preferred reductant is fuel, which can be supplied to the exhaust by injection into engine cylinders during exhaust strokes.
In spite of advances, there continues to be a long felt need for an affordable and reliable exhaust treatment system that is durable, has a manageable operating cost (including fuel penalty), and is practical for reducing NOx emissions from diesel engines to a satisfactory extent in the sense of meeting U.S. Environmental Protection Agency (EPA) regulations effective in 2010 and other such regulations